A radical polymerization method has been a well-known method for polymerizing vinyl monomers to obtain a vinyl polymer. Generally, a radical polymerization method has the disadvantage of the difficulty in controlling the molecular weight of the obtained vinyl polymer. Further, there is the disadvantage that the obtained vinyl polymer is a mixture of compounds having various molecular weights, and thus it is difficult to obtain a vinyl polymer having narrow molecular weight distribution. Specifically, even if the reaction is controlled, the ratio of weight-average molecular weight (Mw) and number-average molecular weight (Mn), (Mw/Mn), can be only reduced to about 2 to 3.
As a method for eliminating the aforementioned disadvantages, since around 1990, a living radical polymerization method has been developed. Specifically, according to the living radical polymerization method, it is possible to control the molecular weight. It is also possible to obtain a polymer having narrow molecular weight distribution. Specifically, a polymer having Mw/Mn of 2 or less can easily be obtained. Therefore, this method has come into the limelight as a method for producing a polymer used in an advanced technology such as nanotechnology.
Catalysts which are currently used in living radical polymerization methods include transition metal complex-type catalysts.
For transition metal complex-type catalysts, complexes in which a ligand is coordinated to a compound having a central metal of Cu, Ni, Re, Rh, Ru, or the like have been used. Such catalysts are described in the following documents for example.
Patent Document 1 (Japanese Laid-open Publication No. 2002-249505) discloses that a complex in which Cu, Ru, Fe, Ni or the like is a central metal, is used as a catalyst.
It should be noted that Patent Document 1 describes in its claim 1 that an organic halide is used as a polymerization initiator. This description is not intended to mean that a halogenated hydrocarbon acts as a catalyst for living radical polymerization. According to the invention of Patent Document 1, a metal complex having a transition metal as the central metal is used as the catalyst for living radical polymerization. According to the invention of Patent Document 1, an organic halide is used as a dormant species that will be described later in the present specification.
Patent Document 2 (Japanese Laid-open Publication No. 11-322822) discloses that a hydrido rhenium complex is used as a catalyst.
It should be noted that Patent Document 2 describes a “catalyst for radical living polymerization comprising a combination of a hydrido rhenium complex and a halogenated hydrocarbon” in claim 1. This description is not intended to mean that a halogenated hydrocarbon acts as a catalyst for living radical polymerization. According to the invention of Patent Document 2, the hydrido rhenium complex is used as the catalyst for living radical polymerization. According to the invention of Patent Document 2, the halogenated hydrocarbon is used as a dormant species that will be described later in the present specification. The combination of the catalyst and the dormant species is described as a catalyst in Patent Document 2, and this does not describe that the halogenated hydrocarbon serves as the catalyst for living radical polymerization.
Non-Patent Document 1 (Journal of The American Chemical Society 119, 674-680(1997)) discloses that a compound in which 4,4′-di-(5-nonyl)-2,2′-bipyridine is coordinated with copper bromide, is used as a catalyst.
It should be noted that Non-Patent Document 1 describes that 1-phenylethyl bromide is used at the time of polymerization of styrene. That is, according to the invention of Patent Document 2, a copper bromide complex is used as a catalyst for living radical polymerization, and 1-phenylethyl bromide is used as a dormant species that will be described later in the present specification.
However, when such a transition metal complex catalyst is used, it is necessary to use a large amount of the catalyst. This is disadvantageous as it is not easy to completely remove the large amount of the catalyst used, from the products after the reaction. Another disadvantage is environmental problems which may occur by the disposal of the catalyst. The transition metal for the living radical polymerization method includes many toxic metals. The disposal of a large amount of such toxic metals causes environmental problems. Furthermore, there are cases where toxicities of catalysts remaining in products cause environmental problems. Due to the toxicity, it is difficult to use the transition metal catalysts for the production of food packages, material for living body, and medical material. Additionally, there is a problem associated with a high electroconductivity of the transition metal remaining in polymer, rendering the polymer conductive and hence unsuitable for use in electronic material such as resist material, organic electrochemical luminescence material, fuel cell, solar cell, lithium-ion cell. Furthermore, the transition metal-type catalysts do not dissolve in a reaction solution unless they form a complex. Therefore, it is necessary to use a ligand as an additive to form a complex. This causes problems, i.e., an increase of the cost of production and also an increase of the total weight of the catalyst used. Further, a ligand is usually expensive and requires a complicated synthesis method. Furthermore, the polymerization reaction requires a high temperature (for example, 110° C. or higher). (For example, in aforementioned Non-patent document 1, the polymerization reaction is performed at 110° C.).
It is noted that a living radical polymerization methods, which do not require a catalyst, have also been known. For example, a nitroxyl-type method and dithioester-type method have been known. However, these methods have the following disadvantages. A special protecting group must be introduced to the polymer growing chain. The protecting group is very expensive. Further, the polymerization reaction requires a high temperature (for example, 110° C. or higher). Further, the produced polymer is likely to have undesirable properties. For example, the produced polymer is likely to be colored differently from the natural color of the polymer. Further, the produced polymer is likely to have an odor.
On the other hand, Non-Patent Document 2 (Polymer Preprints 2005, 46(2), 245-246) and Patent Document 3 (Japanese Laid-open Patent Publication No. 2007-92014) disclose that compounds having Ge, Sn or the like as a central metal are used as catalysts. Patent Document 4 (International Publication WO2008/139980) discloses that compounds having nitrogen or phosphorus as a central metal are used as catalysts.
In regard to the copper complex catalyst described in Non-Patent Document 1, the cost for the catalyst required to polymerize 1 kg of a polymer sums up to approximately several thousand yen. On the other hand, in regard to a germanium catalyst, the cost is cut down to about one thousand yen. Thus, the invention of Non-Patent Document 2 markedly decreases the cost for the catalyst. However, in order to apply living radical polymerization to general-purpose resin products and the like, a further less expensive catalyst is demanded.
In general, it is known that transition metals or compounds of transition metal elements are preferable as catalysts for various chemical reactions. For example, the following is described on page 311 of “Inorganic Chemistry” by J. D. LEE (Tokyo Kagaku Dojin, 1st edition published on Apr. 15, 1982): “Many transition metals and the compounds of the transition metals have catalytic action . . . in some cases, a transition metal may adopt various valences and form unstable intermediate compounds, while in other cases, a transition metal provides good reaction surfaces, and these serve as catalytic actions.” That is, it has been widely understood by those skilled in the art that the properties characteristic to transition metals, such as the ability to form various unstable intermediate compounds, are indispensable in connection with the function of a catalyst.
Furthermore, Ge, Sn and Sb described in aforementioned Non-Patent Document 2 are not transition metals, but are elements that belong to the 4th period or the 5th period of the Periodic Table and have large atomic numbers and have a large number of electrons and a large number of electron orbitals. Therefore, it is surmised in regard to Ge, Sn, and Sb that the fact that these atoms have a large number of electrons and a large number of electron orbitals works advantageously in terms of their action as catalysts.
According to such a common technological knowledge in connection with various catalysts of the prior art, it is believed that the typical elements which belong to the 2nd period and the 3rd period of the Periodic Table, merely have a small number of electrons and a smaller number of electron orbitals, and thus it is disadvantageous to use them in a catalyst compound, and a catalytic action cannot be expected from compounds utilizing these typical elements. In particular, it was believed that compounds having carbon atoms in the center, such as organic compounds cannot be expected to have a catalytic action.
Furthermore, Non-Patent Document 3 discloses a catalyst using a phosphorus compound, but does not describe the use of an organic compound which has an oxidation-reduction capability.
Further, in conventional living radical polymerization, a radical initiator such as a peroxide and diazo compound was used, except for the cases where a transition metal complex is used as a catalyst and nitroxyl is used as a protecting group. It thus has, for example, the following defects:
(1) A radical generated from a radical initiator reacts with a monomer to cause a reaction that is not based on the mechanism of living radical polymerization. As a result, a polymer having a less molecular weight than the desired polymer is mixed in the product, and molecular weight distribution becomes wide.
(2) When block copolymerization is carried out, a homopolymer is mixed in the product. For example, in synthesizing a block copolymer having a structure in which the segment obtained by polymerizing monomer B is linked to the segment obtained by polymerizing monomer A, a reaction of a radical initiator with monomer B produces a homopolymer, and, as a result, the purity of the block copolymer is lowered.
(3) In living radical polymerization, branched polymers which are referred to as, for example, a star-type polymer, and comb-type polymer can be synthesized. When the polymerization of such a branched polymer is carried out, a radical generated from a radical initiator is reacted with a monomer to cause a reaction that is not based on the mechanism of the living radical polymerization, and consequently a linear polymer is mixed in the product.
(4) When surface polymerization is carried out, a polymer that is not bound to a surface is produced. In living radical polymerization, the polymerization of which the starting point is a surface of a solid can be carried out to give a product of which the polymer is bound to the solid surface. In such a case, a radical generated from a radical initiator is reacted with a monomer to cause a reaction that is not based on the mechanism of the living radical polymerization, and thereby a polymer that is not bound to the surface is produced, and the yield is lowered.
Accordingly, a method of performing living radical polymerization without using a radical initiator is desired. In this regard, when the aforementioned catalyst in which a transition metal is the central element is used, living radical polymerization can be performed without using a radical initiator. However, since catalysts in which a transition metal is the central element have the aforementioned defects, it was difficult to utilize it industrially. Non-Patent Document 4 describes methods of using it wherein nitroxyl is used as a protecting group. However, when nitroxyl is used as a protecting group, since there are the aforementioned defects including those defects that a protecting group is very expensive, and the like, it is also difficult to utilize it industrially.